In a case of using a circular accelerator, such as a synchrotron, as a main accelerator of a particle beam therapy system, a technique commonly called “slow extraction” is employed to extract charged particles in the accelerator therefrom. In the “slow extraction”, a beam-dynamically stable region (hereinafter referred to as “separatrix”) is formed in the circular accelerator, and the particles are extracted by controlling the area of the separatrix. The separatrix is ordinarily formed by a multipole electromagnet. The multipole electromagnet is, for example, a sextupole electromagnet. The circular accelerator is described below taking a synchrotron as an example.
While charged particles within the separatrix keep orbiting in the synchrotron, charged particles outside the separatrix cannot keep orbiting stably, resulting in significant deviation of the particle path from the design (central) orbit. Properly designing orientation of the apexes of the separatrix (see FIGS. 3 and 5) for charged particles reaching the boundary of the separatrix to pass through the extraction path allows the charged particles to be extracted from the synchrotron.
Various methods of transferring charged particles within a separatrix toward the boundary thereof have been proposed. In a classical method, strength of a quadrupole electromagnet is varied to change the horizontal tune of charged particles orbiting in the synchrotron, whereby the area of the separatrix is decreased, resulting in transferring the particles toward the boundary of the separatrix. In another method, a radio frequency electric field including a resonance frequency component is applied to the radio frequency electrode to increase the Courant-Snyder invariants of charged particles, thus performing extraction while transferring the particles toward the boundary of the separatrix (for example, Patent Document 1). In still another method, the momentum of charged particles orbiting in the synchrotron is varied during acceleration by the radio frequency accelerating cavity, whereby the tune of the particles is changed and the area of separatrix is decreased, thus performing extraction (for example, Patent Document 2).
In any methods, the invariant parameter called the Courant-Snyder invariant exits for accelerated charged particles, and no unstable extraction occurs in an ideal physical design. In an actual compact synchrotron for a particle beam, however, the Courant-Snyder invariants of charged particles orbiting in the synchrotron increase with elapsed time by being subject to influences of constituent devices, such as ripple noise of the power source for the bending electromagnets and distortion of and noise components of the electric field formed in the radio frequency accelerating cavity.
In other words, a phenomenon occurs in which charged particles that are supposed to exist originally within the separatrix are unexpectedly transferred to the outside of the separatrix owing to disturbance factors. Note that the “elapsed time” here means a time required from injection to acceleration, a waiting time for the extraction enable signal in using for particle beam therapy, or the like.
Before extraction, the area of the separatrix in the phase space is ordinarily set to be significantly larger than the maximum area of the Courant-Snyder invariants of orbiting particles (by no energization of the extraction multipole electromagnet), to keep a condition of no extraction even when there exist disturbance factors. On reception of the extraction enable signal, preparation for extraction is made in which the area of the separatrix is made equal to the maximum value of the Courant-Snyder invariants of the charged particles by a method such as for energizing, for example, the sextupole electromagnet, to start gradually the extraction. In a case where a Courant-Snyder invariant distribution of the charged particles extends over a wider range than an estimate Courant-Snyder invariant distribution owing to disturbance factors so far, however, a much larger number of particles per unit time than ordinary slow extraction—this is referred to as “spike”—may extracted in an preparation period for the extraction only by slightly decreasing the separatrix area. Moreover, characteristics (width and momentum) of the beam extracted in the period may in some cases differ significantly from those in the design condition.
In a scanning irradiation proposed for particle beam therapy systems recently applied to cancer treatment (for example, Patent Document 3), temporal stability of the intensity waveform of a particle beam used for the irradiation is highly requested more than before because the irradiation needs to be controlled spot by spot. A particle beam, i.e., charged particles extracted during the above-described spike period, has an extreme peak intensity, thus causing irradiation doses to be uncontrollable. Moreover, the particle beam is of poor quality, for example, its parameters, such as a beam diameter, are different from the design parameters. Since the particle beam during the spike period is thus uncontrollable for the irradiation, it is desired not to irradiate a target with such a beam.
Techniques for removing the spike has been proposed, in which the particle beam extracted from the accelerator during the spike period is removed, for example, at a damper by being deflecting the traveling direction so as not to irradiate a target or the particle beam is removed in the accelerator by narrowing the separatrix (stable region) during a preparation period for extraction so as not to be extracted from the accelerator (for example, Patent Document 4).